MAPT and Alzheimer disease: Finally, we used bacterially expressed MTBD-tau as target, thereby excluding full-length, other isoforms (such as big tau, a high molecular weight (~110 kDa) tau isoform that includes exon 4a in the MAPT transcript [49]) or post-translationally modified tau epitopes, like the phosphorylated forms p-tau181 [50] and p-tau217 [51], that are increasingly used as biomarkers for AD diagnosis.